The present invention relates to a full duplex transceiver comprising a hybrid coil and automatic impedance adjustment means for use in a facsimile system, audio or video communication system, data transfer system or the like.
Full duplex transmission is well known in the art. Data signals of one frequency are transmitted from a local transceiver to a remote transceiver over a transmission line. Data signals of a different frequency transmitted by the remote transceiver are received by the local transceiver over the same transmission line. Each transceiver comprises a transmission unit for generating signals for transmission, a reception unit for processing received signals and a hybrid coil connecting the transmission and reception units to the transmission line. The hybrid coil is also known in the art as a bridge transformer and functions to pass transmitted signals from the transmission unit to the transmission line while preventing received signals from reaching the transmission unit. The hybrid coil also functions to pass received signals from the transmission line to the reception unit while preventing transmitted signals from reaching the reception unit.
In actual practice, the hybrid coil is not capable of completely preventing the transmitted signals from passing therethrough to the reception unit and there is a certain leakage of transmitted signals through the hybrid coil which reach the reception unit. This problem is generally overcome by providing a frequency selective filter between the hybrid coil and the reception unit. Typically, such a filter is embodied by a bandpass filter which passes only the received signals or a high or low pass filter. A high pass filter is used when the frequency of the received signals is higher than the frequency of the transmitted signals and rejects all frequencies below the frequency of the received signals. A low pass filter is used when the frequency of the received signals is lower than the frequency of the transmitted signals and rejects all frequencies above the frequency of the received signals.
Whereas such a filter generally attenuates the transmission leakage signals to a tolerable level, further problems are involved in a full duplex transceiver system of this type in that the performance of the hybrid coil is substantially influenced by the impedance of the transmission line as viewed from the hybrid coil. Where the impedance is of the proper value, generally on the order of 600.OMEGA., and is constituted by pure resistance, the operation of the hybrid coil will be optimum. The rejection of transmitted leakage signals will be maximum, frequency response will be flat and there will be minimal phase distortion. However, as the impedance deviates from the optimum value the performance of the hybrid coil will become seriously degraded. Rejection of transmitted leakage signals will become insufficient and there will be substantial frequency and phase distortion of both the transmitted and received signals. As the transmission line is typically a public telephone line or the like having an impedance which varies over a considerable range and has an unpredictable value at any particular time, the quality of full duplex data transmission has heretofore ranged from excellent to unacceptable and has been beyond control.